Antibody therapy has been established for the targeted treatment of patients with cancer, immunological and angiogenic disorders. The use of antibody-drug conjugates (ADC), i.e. immunoconjugates, for the local delivery of cytotoxic or cytostatic agents, i.e. drugs to kill or inhibit tumor cells in the treatment of cancer theoretically allows targeted delivery of the drug moiety to tumors, and intracellular accumulation therein, where systemic administration of these unconjugated drug agents may result in unacceptable levels of toxicity to normal cells as well as the tumor cells sought to be. Maximal efficacy with minimal toxicity is sought thereby. Efforts to design and refine ADC have focused on the selectivity of monoclonal antibodies (mAbs) as well as drug-linking and drug-releasing properties. Both polyclonal antibodies and monoclonal antibodies have been reported as useful in these strategies.
Amatoxins
Amatoxins are cyclic peptides composed of 8 amino acids. They can be isolated from Amanita phalloides mushrooms or prepared from the building blocks by synthesis. Amatoxins specifically inhibit the DNA-dependent RNA polymerase II of mammalian cells, and thereby also the transcription and protein biosynthesis of the affected cells. Inhibition of transcription in a cell causes stop of growth and proliferation. Though not covalently bound, the complex between amanitin and RNA-polymerase II is very tight (KD=3 nM). Dissociation of amanitin from the enzyme is a very slow process what makes recovery of an affected cell unlikely. When the inhibition of transcription lasts too long, the cell will undergo programmed cell death (apoptosis).
Conjugates of Amatoxins and Target-binding Moieties
Earlier patent application EP 1 859 811 A1 (published Nov. 28, 2007) by the inventors describes conjugates, in which β-amanitin is coupled to albumin or to the monoclonal antibodies HEA125, OKT3, and PA-1. Furthermore, the inhibitory effect of these conjugates on the proliferation of breast cancer cells (MCF-7), Burkitt's lymphoma cells (Raji), and T-lymphoma cells (Jurkat) was studied.
Epithelial Cell Adhesion Molecule (EpCAM) Antigen
Epithelial cell adhesion molecule (EpCAM, CD326) is one of the best-studied target antigens on human tumors (Trzpis et al., 2007; Baeuerle and Gires, 2007). It represents a type I membrane glycoprotein of 314 amino acids with an apparent molecular weight of 40 kDa (Balzar et al., 1999). It is overexpressed in the majority of adenocarcinomas (Winter et al., 2003; Went et al., 2004). In particular, EpCAM expression is enhanced in node-positive breast cancer, epithelial ovarian cancer, cholangiocarcinoma, pancreatic adenocarcinoma and squamous cell head and neck cancer. Increased EpCAM expression is indicative for a poor prognosis in breast and gallbladder carcinomas (Gastl et al., 2000; Varga et al., 2004; Spizzo et al., 2002; Spizzo et al., 2004). Importantly, EpCAM is expressed by tumor initiating or cancer stem cells in mammary, colorectal and pancreatic carcinomas (Al-Hajj et al., 2003; Dalerba et al., 2007; Li et al., 2007).
EpCAM-specific monoclonal antibodies have been used as a diagnostic tool for the detection of rare circulating tumor cells in cancer patients (Allard et al., 2004; Nagrath et al., 2007). A couple of engineered anti-EpCAM antibodies are currently investigated in clinical studies.
HER2 Antigen
HER2 (Her2/neu; ErbB2), a receptor tyrosine kinase with an apparent molecular weight of 185 kDa is overexpressed in about 25-30% of human breast cancers and gastric cancers. This overexpression, which is often due to amplification of the receptor-encoding gene, generally represents a poor prognosis, often involving progressive disease in the years after the initial diagnosis is made.
Monoclonal antibody therapy has been established for the targeted treatment of patients with Her2/neu-positive cancers. HERCEPTIN® (trastuzumab) is a recombinant DNA-derived humanized monoclonal antibody that selectively binds with high affinity in a cell-based assay (Kd=5 nM) to the extracellular domain of the human epidermal growth factor receptor 2 protein, HER2 (ErbB2). Trastuzumab is an IgG1 kappa antibody that contains human framework regions with the complementarity-determining regions of a murine antibody (4D5) that binds to HER2. Trastuzumab binds to the HER2 antigen and thus inhibits the growth of cancerous cells. Because trastuzumab is a humanized antibody, it minimizes any HAMA response in patients. Trastuzumab has been shown, in both in vitro assays and in animals, to inhibit the proliferation of human tumor cells that overexpress HER2. Trastuzumab is a mediator of antibody-dependent cellular cytotoxicity, ADCC. HERCEPTIN® is clinically active in patients with ErbB2-overexpressing metastatic breast cancers that have received extensive prior anti-cancer therapy. Although HERCEPTIN® is a breakthrough in treating patients with ErbB2-overexpressing breast cancers that have received extensive prior anti-cancer therapy, the majority of the patients in this population fail to respond or respond only poorly to HERCEPTIN® treatment. Therefore, there is a significant clinical need for developing further HER2-directed cancer therapies for those patients with HER2-overexpressing tumors or other diseases associated with HER2 expression that do not respond, or respond poorly, to HERCEPTIN® treatment.